Missing sudo in installation instructions for Ubuntu

Reported by user:

When in https://tails.boum.org/install/debian/usb/index.en.html, step 2/6 Install Tails Installer for Ubuntu, there is no sudo or Root terminal mentioned, so when users do

apt install tails-installer

They get stuck with the error

Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?*

Please review branch.

Feature Branch: emmapeel:doc/15552-sudo_in_apt_instructions

Related issues

Blocks #15411 (closed)

Original created by @emmapeel on 15552 (Redmine)

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